For various reasons, it is often necessary to insure that the underside of a kiln car is maintained in a relatively cooler atmosphere than the upper deck side (on which uncured ceramic materials are typically carried for firing in the kiln). For example, it may be necessary to achieve sufficient cooling to avoid overheating of mechanisms such as kiln car wheels or the like located beneath the deck of a kiln car.
In general, the foundation and lower side walls of the tunnel kiln may be cooled in an attempt to dissipate heat flowing through the deck of kiln cars to avoid such overheating. One typical prior art practice is to attempt cooling of the undercar channel by means of air drawn through the undercar channel from the exit end of the tunnel kiln toward its entrance end. The firing channel of the tunnel kiln is typically also flushed with gases flowing the same direction so that a pressure gradient develops in both channels (i.e., the firing channel located above the car decks and the undercar channel located therebelow) from the exit end towards the entrance end of the tunnel.
However, because there are different gas flow resistances in the two different channels, the pressure gradient is different as a function of distance along the tunnel thereby leading to "false" air flows between the two channels. Such air flows between the two channels is a situation which must be avoided by appropriate measures so as to avoid undue heating of the undercar channel (or undue cooling of the firing channel).
In my related parent application Ser. No. 903,924 filed Nov. 17, 1986, now U.S. Pat. No. 4,722,682, I describe a system which permits controlled cooling along a tunnel kiln utilizing convection currents in divided cooling sub-chambers formed along the length of the undercar channel. By blocking the longitudinal flow of air along the undercar channel, it is possible to permit local pressure equalizations vertically to the upper firing channel and thus reduce the need for the traditional horizontal pressure seal along the tunnel length.
According to my other related U.S. Pat. No. 4,744,750 problems which may arise when an existing tunnel kiln is retrofitted with cooling sub-chambers are alleviated by a system which accomplishes undercar cooling near the end of the firing zone and in the cooling zone of a tunnel kiln. Thus, the improved undercar cooling system of U.S. Pat. No. 4,744,750 can be economically retrofitted to existing tunnel kilns--while still reducing the requirement for a traditional horizontal seal arrangement.
It will be understood that the sides of kiln cars typically travel in close proximity to the tunnel side walls. One conventional method for sealing the moving kiln car sides to the kiln side walls is to provide aprons along the longitudinal car sides which dip into sand filled channels in skirting of the kiln side walls such that the sand forms a closed barrier extending the length of the kiln. Transverse joints between successive kiln cars may be sealed by means of conventional elastic material cords (e.g., see Lingl Leaflet F045/3 dated July 1982).
Although the purpose of such prior sand barriers is to substantially prevent pressure equalization between the undercar channel and the firing channel, it is far from a perfect seal. In the first place, for design and cost reasons, the depth to which such aprons dip into the sand must be comparatively small. In addition, the sand must be made comparatively course so that it will be heavy enough so as not to be blown out of the barrier area and entrained in the moving gas flows. As a result, the sand barrier actually is permeable to gas and provides a far from perfect seal.
Another prior approach (EP OS 0, 086,693) uses a kiln car including a box-like structure open at the bottom and provided with sheet metal aprons extending all about the car. At the entrance to the tunnel kiln, each car is lowered into a running fluid bath which provides a continuous hydraulic seal below the car train. The fluid is circulated under the cars for cooling purposes. However, in addition to requiring lowering and lifting devices for each kiln car at the entrance and exit of the tunnel kiln, the cooling fluid is entrained in one large continuous container so that it is not possible to control the degree of cooling as a function of position along the tunnel kiln. For example, heat is undesirably removed even in the initial heat-up zone of the tunnel kiln where undercar cooling is neither necessary nor desirable (e.g., because it ultimately removes heat from the firing channel which, at this point, is contrary to the desired purpose of getting the top-side of the car and material carried thereon up to kiln curing temperatures as fast as possible).
It is also known to provide multiple trains of kiln cars travelling parallel to one another in side-by-side fashion through an appropriately configured multilane tunnel kiln having no intermediate longitudinal walls disposed between adjacent kiln car trains. Such conventional kiln cars have typically been equipped with the same conventional aprons described above which dip into sand-filled channels disposed laterally of each train. The problems associated with this conventional "sand trough" sealing technique for a single train of kiln cars increases when multiple kiln car trains are employed. For example, greater lateral distances are needed between adjacent kiln car trains so as to accommodate the required volume of sand in the channels to seal each of the multiple trains as effectively as for a single train. This undesirably disrupts the gas flow conditions existing in the firing channel. Furthermore, the increased number of sand-filled channels in multitrain kiln tunnels accentuates the "leaks" between the firing channel and the undercar channel that are present with a single train kiln thereby rendering it more difficult to heat the material on the kiln cars to the desired kiln temperature in the heating zone. For these reasons, multitrain tunnels have not, in practice, been constructed for quite some time.
It would therefore be quite advantageous if a multitrain kiln tunnel was provided which minimizes "leaks" between the firing and undercar channels while yet capable of providing undercar cooling in accordance with my earlier patent applications.
I have now discovered a multitrain kiln tunnel which provides an almost perfect seal between the firing and undercar channels without resort being made to intermediate continuous walls positioned between adjacent kiln car trains and which permits the distance between adjacent kiln car trains to be maintained at an acceptably small dimension. Furthermore, my novel multitrain kiln tunnel facilitates the use of the more narrow kiln cars customarily associated with older multitrain tunnel kilns with minimal conversion thereof.
According to this invention, there is provided a multilane tunnel kiln having a longitudinal pedestal positioned between adjacent kiln car trains. The kiln cars themselves are provided with depending aprons on their leading and/or trailing faces so that little (e.g. 10 millimeters or less) clearance, if any, exists between the aprons, on the one hand, and the bottom of the tunnel kiln and laterally adjacent pedestals (or a laterally adjacent pedestal and the kiln wall), on the other hand. Thus, undercar cooling can now be accomplished in a multitrain tunnel kiln employing the techniques of my earlier U.S. Pat. No. 4,744,750--that is, employing a continuous undercar channel (e.g., depressed continuously below such car aprons) in the latter half of the undercar channel (i.e., from mid-firing zone through the cooling zone).
If desired, the depressed undercar channel provided only along the latter portion of the kiln may include plural sub-chambers disposed therealong beneath the intended track of the multiple trains of kiln cars (i.e., similar to the system described in my earlier U.S. Pat. No. 4,722,682). Gas flows to and from each of the sub-chambers in this alternate embodiment are substantially isolated and prevented except for that required to equalize pressure between the sub-chamber and the section of the firing channel located directly thereabove. A heat exchanger provides cooling within the sub-chamber thus setting up convection currents within the sub-chamber which tend to cool the underside of the kiln car located directly thereabove. Individual sub-chambers may be cooled differently as a function of prevailing temperature. However, since there is no substantial gas flow into or out of the sub-chamber, there is no substantial leakage gas flow between the cooling channel and the firing channel. This alternate embodiment thus generally utilizes the techniques described in my copending parent application Ser. No. 930,924, now U.S. Pat. No. 4,722,682.
Preferably, a radiation blocking structure is employed in the pressure equalization passage located between the cooling channel and the firing channel so as to prevent direct radiation transfers of heat energy from the firing channel into the cooling channel.
The advantages of this invention can be achieved, at least in part, because intensive undercar cooling is limited to the latter portion of the kiln (e.g., mid firing zone through cooling zone) where expected undercar temperatures make such cooling necessary--while minimizing inter-channel leakage in the initial portion of the kiln (e.g., the heat-up zone).
As a result, sand filled channels or other types of attempted "perfect" seals between the firing channel and the undercar cooling channel are no longer required thereby permitting, in a multitrain kiln, the kiln trains to be closely positioned adjacent one another in the kiln. And, as an added advantage, the undercar channel can be cooled at different rates in different sections or sub-chambers as a function of position along the tunnel kiln. Furthermore, the kiln cars can continue to be transported along a rail system in the same plane both inside and outside the tunnel kiln so that lifting or lowering devices are avoided. This greatly facilitates movement and circulation of the kiln cars with conventional apparatus and existing facilities which can be retrofitted or converted after the fact to practice the present invention.